The invention relates to the manufacture of coiled tubing. More particularly, the invention relates to an improved weld for continuous coiled tubing.
Coiled metal tubing has many applications in the petroleum industry. For example, coiled metal tubing can be used to insert high pressure fluids into a well, to guide measuring instruments into a well, and in many instances for oil well drilling, production, and flow line applications. Typically, the coiled metal tubing is stored on a large reel from which it can be uncoiled for insertion into the well and recoiled when the operation is complete.
Metal tubing is formed by bending flat strip into tubing and welding along the longitudinal seam. Because the length of strip commercially available is limited, however, the length of metal tubing which can be formed by this process is also limited. Thus, assuming the maximum length of strip available in the desired size is 4,000 feet, if a 20,000 foot length of coiled metal tubing is desired, according to the prior art it would be necessary to butt weld five 4,000 lengths of coiled tubing formed from five lengths of strip.
Butt welding two lengths of metal tubing forms a weld such as that shown in FIG. 1. The weld has a ring of weldment 1 and a heat-affected zone 3A, 3B on each side thereof. Because the heat-affected zones are unable to withstand repeated coiling and uncoiling stresses, the composite 20,000 length of coiled metal tubing would typically break at one of the butt welds after it had been coiled and uncoiled several times. This breakage would, at the very least, require rewelding of the tubing, and frequently would require fishing a length of tubing from the bottom of the well. Also, the ring of weldment 1 shown in FIG. 1 extends inwardly so as to partially block the inner diameter of the tubing. In addition to interrupting fluid flow through the tubing, this decrease in inner diameter tends to obstruct passage of measurement devices sent through the tubing.
To overcome the disadvantages in the prior art discussed above, the assignee of the present invention developed the helical weld shown in FIG. 2, which is described in detail in U.S. Pat. Nos. 4,863,091 and 5,191,911, the disclosures of which are hereby incorporated by reference. To form the helical weld, two lengths of strip 5A, 5B are cut at complimentary acute angles (preferably 45.degree.) as shown in FIG. 3a. The strips are then butted together, and are welded along the diagonal seam (see FIG. 3b). To provide excess weldment at the edges of the joined strip and to prevent burnout at the edges of the strips, tabs 7A, 7B may be tack welded to the edges of the strips prior to welding, and a welding head 9 is moved from the outer edges of the first tab 7A along the diagonal joint to the outer edge of the second tab 7B. Thus, a weld is formed which is perpendicular to the top and bottom surfaces of the strips (see FIG. 3c) and at an acute angle to the longitudinal direction Z thereof.
Once the weld is completed, the tabs 7A, 7B are removed, and the top, bottom, and side surfaces in the vicinity of the weld are finished such that the resulting weld has dimensions identical to the dimensions of the original strip.
When the composite strip formed from the two strips 5A, 5B is then formed into tubing as shown in FIG. 2, the weld 11 takes on a helical shape which begins and ends at the longitudinal seam 13. To allow time for completion of the angled weld and finishing steps in the tube mill, an accumulator may be used as described in the prior patents mentioned above. Alternatively, the composite strip can be formed to a desired length and coiled onto a payoff wheel, from which it may then be unreeled at a constant rate for formation of tubing.
As a result of the prior art weld, the inner and outer diameters of the tubing in the vicinity of the weld 11 are uniform. Thus, the weld does not obstruct flow of fluid through the tubing or movement of measuring devices. In addition, no heat-affected zones are formed and the helical weld is able to withstand repeated coiling and uncoiling stresses.